Field of the Invention
The present invention relates to a work vehicle, particularly to a hybrid type work vehicle and a control method thereof.
Background Information
Among work vehicles, such as a wheel loader, a type of work vehicle equipped with a power transmission device including a torque converter and a multistage gearbox (hereinafter referred to as “a torque converter type transmission device”) has been heretofore known. On the other hand, in recent years, a hybrid type work vehicle configured to travel by a driving force from an engine and that from a motor has been proposed as a type of work vehicle equipped with a type of power transmission device replacing the torque converter type transmission device. For example, Japan Laid-open Patent Application Publication No. 2006-329244 discloses an HMT (hydraulic-mechanical transmission) or an EMT (electro-mechanical transmission) as the power transmission device for the hybrid type work vehicle.
The HMT includes a planetary gear mechanism and both of a first pump/motor and a second pump/motor that are connected to rotary elements of the planetary gear mechanism. Each of the first pump/motor and the second pump/motor is configured to function as either a hydraulic motor or a hydraulic pump in accordance with a travelling condition of the work vehicle. The HMT is configured to be capable of seamlessly changing the rotational speed of an output shaft by changing the rotational speeds of the pumps/motors.
The EMT uses electric motors instead of the hydraulic motors used in the HMT. Specifically, the EMT includes a first generator/motor and a second generator/motor. Each of the first generator/motor and the second generator/motor is configured to function as either an electric motor or a generator in accordance with a travelling condition of the work vehicle. Similarly to the HMT, the EMT is configured to be capable of seamlessly changing the rotational speed of an output shaft by changing the rotational speeds of the generators/motors.